Various techniques of etching resist-imaged photomasks, silicon wafers or other semiconductor materials have been used in semiconductor fabrication processes. A wet etching technique conducted in an immersion tank is a practical high-throughput, flexible fabrication process. By properly selecting etchant chemicals, etch reactions with the target film are thermodynamically favored over reactions with other films. Desirable etch-rate ratios can usually be obtained.
A wet etching method is especially suitable for the blanket etching of polysilicon, oxide, nitride and metal. The method is capable of providing the necessary etch selectivity, a damage-free interface and particle-contamination-free wafers. In more recently developed wet etching technology, automated robotic handling systems and ultra-pure chemicals have been used to further improve particle control and process consistency. A well-controlled wet etching technique is therefore the choice of etching process in VLSI and ULSI fabrication processes.
One of the key criteria in carrying out a wet etching process is that the etch products must be soluble in the etchant solution and therefore, no contaminating particles are generated. In an immersion etching process, the volume of the etching tank should be large enough to create enough pressure on the wafer surface in order to dislodge hydrogen gas bubbles evolved during etching reactions; to ensure an accurate balance of the etchant components; to keep the concentration of the etchant relatively constant; and to reduce the number of times the etchant tank must be changed in a production environment. An etchant bath change creates expensive down time, and furthermore, the handling of highly hazardous corrosive materials should be minimized from a safety standpoint.
Wet etching is a frequently used technique for stripping photoresist films from silicon wafers where a complete removal of the resist images without adversely affecting the wafer surface is desired. The resist layer or images should be completely removed without leaving any residues, including contaminant particles that may have been present in the resist. The underlying surface of the photoresist layer should not be adversely affected, for instance, undesirable etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce reasonable bath yield in order to prevent redeposition of dissolved resist on the wafers. The etchant should completely dissolve the photoresist layer in a chemical reaction, and not just lifting or peeling so as to prevent redeposition. It is also desirable that the etching or stripping time should be reasonably short in order to permit a high wafer throughput.
Wet chemical cleaning process is also a form of wet etching process for electronic substrates. A most commonly used wet chemical cleaning technique is based on hot alkaline or acidic hydrogen peroxide (H.sub.2 O.sub.2) solutions. The mixture is commonly used to remove chemically bonded films from the surface of an electronic substrate prior to critical processing steps. A frequently used cleaning process is known as the RCA cleaning technique which is based on a two-step process of a Standard Clean-1 (or SC-1) followed by Standard Clean-2 (or SC-2). Both the SC-1 and the SC-2 cleaning solutions incorporate the strong oxidizing capability of hydrogen peroxide. Specifically, SC-1 is an aqueous alkaline solution that readily removes organic films. For instance, SC-1 etches thermally grown oxide at a rate of about 0.1 nm/min. SC-1 is typically a 1:1:5 solution mixture of ammonium hydroxide (NH.sub.4 OH, at 27% concentration), unstabilized H.sub.2 O.sub.2 (at 30% concentration) and DI water. The SC-1 cleaning solution is very effective in removing organic contaminants since NH.sub.4 OH readily dissolves organic films.
A typical wet chemical cleaning system 10 is shown in FIG. 1. The conventional system has a wet chemical cleaning tank 12 which includes an inner tank 14 and an outer tank 16. The outer tank 16 is positioned outside the inner tank 14 for providing overflow protection. This allows an overflow protection where the inner tank 14 is usually filled with a cleaning solution through inlet 18. The wet cleaning tank 12 is further positioned inside a safety containment tank 20 for preventing accidental spill of the caustic or acidic solution that are normally used in the cleaning tank 14. The safety containment tank 20 further provides containment to rinse tanks 22 and 24. The rinse tanks 22, 24 are normally filled with DI water or any other suitable rinse solution for pre-rinse and post-rinse, respectively before and after the cleaning process in tank 14. The wet cleaning tank 12 may further include a filter system and a heater system which are not shown in FIG. 1.
A major component of the SC-1 cleaning solution, i.e., an aqueous solution of ammonia, or ammonium hydroxide of 29% concentration is stored in a supply tank 30. The aqueous solution of ammonia 28 is pumped by a recirculating pump 34 into the inner tank 14 through outlet 18. The flow of the aqueous solution of ammonia 28 is controlled by a flow control valve 36 provided in the flow conduit 32. A safety overflow conduit 38 is also provided in fluid communication with the supply tank 30 to prevent the supply tank 30 from being over-pressured.
In operation, an electronic substrate such as a silicon wafer is first pre-rinsed in the rinse tank 22, then cleaned by soaking in the wet cleaning tank 14 for a predetermined period of time. The electronic substrate is then removed from tank 14 and dipped into the post-rinse tank 24 filled with DI water for rinsing off the cleaning solution. In the conventional set-up shown in FIG. 1, the safety containment tank 20 which surrounds the rinse tanks 22, 24 and cleaning tank 12 is substantially isolated from the atmosphere for safety reasons. This presents a problem in that through the outlet 42 of the safety overflow conduit 38, vaporized ammonia frequently escapes to enter the safety containment tank 20. The ammonia vapor 40 fills the chamber interior 26 of the safety containment tank 20.
Since, aqueous solution of ammonia has a relatively low boiling temperature, i.e., at approximately 32.degree. C., aqueous ammonia vaporizes easily at temperatures higher than 32.degree. C. The aqueous solution of ammonia 28 in the supply tank 30 is normally kept at an elevated temperature of 70.degree. C. to facilitate mixing with the other two components of the SC-1 cleaning solution, i.e., the H.sub.2 O.sub.2 and DI water. The mini-environment 26 that is present in the safety containment tank 20 is normally maintained at a temperature around 55.degree. C. The surrounding temperature where the aqueous solution of ammonia 28 is kept is therefore substantially higher than 32.degree. C. which causes the aqueous ammonia to vaporize and to escape from outlet 42 of the safety overflow conduit 38 to fill the interior chamber 26 of the safety containment tank 20.
When the chamber interior 26 is filled with ammonia vapor, at the instant that a silicon wafer is being removed from the SC-1 tank 12 into the chamber interior 26, ammonia vapor 40 attaches itself onto the surfaces of the silicon wafer (not shown). When the wafer is again dipped into the post-rinse tank 24 that is filled with DI water, the ammonia vapor attached on the wafer surface reacts with DI water thus forming aqueous solution of ammonia. When the aqueous solution of ammonia is concentrated at localized spots on the wafer surface, a defect of "silicon hole" occurs on the surface of the wafer. The silicon hole is caused by the removal, or the wet etching, of silicon oxide from the wafer surface by the concentrated aqueous ammonia at localized spots. The conventional wet cleaning equipment 10, as shown in FIG. 1, is not provided with cooling devices and thus the ammonia vapor generation cannot be stopped or minimized.
It is therefore an object of the present invention to provide a method for preventing defect formation on a silicon wafer processed in a wet cleaning system that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for preventing defect formation on a silicon wafer processed in a wet cleaning system which utilizes aqueous solution of ammonia as one of the components in the cleaning mixture.
It is a further object of the present invention to provide a method for preventing defect formation on a silicon wafer processed in a wet cleaning system by providing cooling means to a conduit which connects a chemical supply tank and a cleaning tank.
It is another further object of the present invention to provide a method for preventing defect formation such as silicon holes on a silicon wafer processed in a wet cleaning system by cooling at least one conduit connecting a supply tank and a cleaning tank to a temperature that is sufficiently low such that formation of ammonia vapor in the at least one conduit is prevented.
It is still another object of the present invention to provide a method for preventing defect formation on a silicon wafer processed in a wet cleaning system by cooling a conduit connecting between a supply tank and a cleaning tank with a cooling water having a temperature of less than 20.degree. C.
It is yet another object of the present invention to provide a method for preventing defect formation on a silicon wafer processed in a wet cleaning system by cooling a conduit connecting a supply tank and a cleaning tank such that ammonia vapor is not generated to form aqueous ammonia for attacking the surface of the silicon wafer.
It is still another further object of the present invention to provide a wet cleaning system for an electronic substrate which utilizes a supply tank for holding an aqueous solution of ammonia and a cleaning tank for receiving the aqueous solution of ammonia through a conduit that is sufficiently cooled for preventing ammonia vapor from escaping into the environment that surrounds the cleaning tank.
It is yet another further object of the present invention to provide a wet cleaning system for an electronic substrate which includes a supply tank, a cleaning tank, a conduit connecting the supply tank and the cleaning tank and a cooling means for cooling the conduit by a coolant having a temperature of not higher than 20.degree. C.